1. Field of the Invention
This invention relates generally to superabrasive cutting elements used in rotary drill bits, also referred to as drag bits, for use in drilling subterranean formations. More specifically, the present invention pertains to superabrasive cutting elements securable to rotary drill bits in a manner which minimizes unwanted stresses in the superabrasive member, particularly when the superabrasive cutting element is positioned at a high positive rake angle.
2. Background of the Invention
Superabrasive material such as polycrystalline diamond compact (PDC) and cubic boron nitride are commonly used in the fabrication of cutting elements employed in drill bits, particularly drill bits which are relied upon by the oil and gas industry for drilling wells in formations of earth in the exploration and production of oil and gas. Such superabrasive material may be formed into the bit body as a self-supporting member or may be employed in cutting elements which comprise a table or layer of superabrasive material joined to a substrate, or backing, of the cutting element. Typically, such cutting elements, such as representative PDC cutting element 214 depicted in cross-section in FIG. 2A, comprise a substantially planar superabrasive, or polycrystalline diamond table, such as table 216, which is disposed on an underlying supportive substrate, or backing, 218 of a suitably strong material such as tungsten carbide (WC) or carbides mixed with other metals in which the diamond table is sintered or bonded to the substrate by methods known within the art. Superabrasive diamond table 216 typically will have a planar, generally circular cutting surface 226, as can be seen in FIG. 2B which is a top view of cutting element 214. As can be seen in FIGS. 2A and 2B, cutting element 214 is provided with a cutting surface 226 which is generally planar or flat in that it extends in only two directions or dimensions, and wherein the cutting surface itself does not extend in a third direction or dimension so as to provide cutting surface 226 with a nonflat or curved cutting surface. A superabrasive cutting element of this type is commonly known as a polycrystalline diamond compact cutter or PDC cutter.
A conventional cutting element, such as a PDC cutter, is positioned in the body of the drill bit so that the superabrasive material contacts and engages subterranean formations for cutting the formation as the drill bit is rotated by the drill string, or alternately a downhole motor in which it is connected. Several factors can contribute to how efficient or inefficient the cutting element performs. Traditionally, cutting elements such as PDC cutters are positioned on the bit body of a drill bit to have either a positive rake angle, zero rake angle, or a negative rake angle with respect to the formation to be engaged by the cutter as the bit rotates and proceeds into the formation being drilled. This terminology of positive, zero, and negative rake angles as used within the art in describing the rake angle of a given cutter is illustrated in FIG. 1. Representative PDC cutters 200, 208, and 214 are all generally cylindrical in configuration and are each provided with respective superabrasive or diamond tables 202, 210, and 216 mounted on respective substrates 204, 212, and 218. Each of the cutters are designed and positioned to laterally engage the formation in the direction of arrow 206. Cutter 200 is regarded as having a positive rake angle due to cutting surface 222 of superabrasive table 202 thereof being inclined at an angle exceeding 90xc2x0 with respect to formation 220 as illustrated. Thus, as the angle becomes more obtuse, or approaches 180xc2x0, it is regarded as being more xe2x80x9cpositivexe2x80x9d. Cutter 208 is regarded as having 0xc2x0 rake angle due to cutting surface 224 of superabrasive table 210 being generally perpendicular to formation 220. Lastly, cutter 214 is regarded as having a negative rake angle due to cutting surface 226 of superabrasive table 216 being inclined less than 90xc2x0 with respect to formation 220 as illustrated. Thus, as the angle becomes more acute, or approaches 0xc2x0, it is regarded as being more xe2x80x9cnegativexe2x80x9d.
The characteristics of the formation being cut further influence the choice of cutting element design and placement on the body of the drill bit. For example, a PDC cutter is subjected to significant tangential loading as the drill bit rotates. Additionally, it is known that positioning the cutting element with a negative rake angle places the formation in compression. Contrastingly, positioning the cutting element with a positive rake angle results in the formation being placed in tension as the formation is engaged and cuttings or chips are sheared therefrom.
Further, it is known that conventional PDC cutter performance can be compromised by residual stresses which are induced within the cutting element itself and particularly in the area of the interface, designated as 228 in FIG. 2A, where the planar diamond table is joined with the substrate. That is, while the superabrasive diamond table is generally in compression and the substrate in tension, conventional PDC""s display an undesirable amount of residual stress around the interface between the diamond table and the substrate, which stress is principally caused by different coefficients of thermal expansion in the diamond and the substrate. The high loading imposed on conventional PDC cutters during drilling, in combination with the residual stress, is known to cause unwanted spalling and delamination of the diamond table from the substrate.
Attempts have been made to remedy or lessen the failure of cutting elements employing PDCs during drilling by modifying or redirecting the residual stresses in PDC cutters by way of varying the configuration of PDC cutters. Examples of such efforts to modify the stresses in PDC""s by modifying the configuration of the diamond table, the substrate, or both, are disclosed in U.S. Pat. No. 5,435,403 to Tibbitts, U.S. Pat. No. 5,492,188 to Smith, et al., and U.S. Pat. No. 5,460,233 to Meany, et al. Another type of improvement in drill bit design is disclosed in U.S. Pat. No. 5,437,343 to Cooley, et al., which discloses the use of multiple chamfers at the periphery of a PDC cutting face to enhance the resistance of the cutting element to impact-induced fracture.
It is known that conventional superabrasive cutting elements can be positioned in the bit body in a manner which optimizes cutting ability under the loading conditions of a particular formation. That is, the type of rock in the formation, the rock stresses, the filtration and the bit profile may all contribute to the performance of the cutting element. It has also been recognized that the location of the cutting element on the bit body influences the capability of the cutting element to withstand certain loading stresses. For example, it has been noted that a conventional planar cutting element located on the bit flank or shoulder may typically experience greater tangential loading than a cutting element located on the bit nose or bit gage. Further, positioning the cutting element in the bit body with a back rake (usually negative back rake) enables the cutting element to better withstand loading forces imposed upon it during drilling operations and lessens failure of the cutting element.
However, while a higher effective negative back rake permits the use of conventional planar PDC cutters, such higher effective back rakes reduce the aggressiveness of the cutter. This factor can be critical in cutting elements which are located on the bit flank or shoulder where the greatest amount of cutting of the formation occurs. Thus, it would be advantageous to provide a cutting element which is configured to effectively and aggressively cut a given earthen formation while being positioned at a high positive rake angle to place the formation in tension, thereby maximizing cutting performance and cutter durability, and it would be advantageous to position the cutting element in a manner which enhances compressive loading of the cutting element and reduces tensile stresses within the superabrasive cutter during operation of the drill bit.
Further, it would be an advantage in the art to provide means for removing the material cut from the formation as the cutters are acting upon the formation. One means of removing cut material is disclosed, for example, in U.S. Pat. No. 5,199,511 to Tibbitts, et al., wherein the cutters xe2x80x9cshearxe2x80x9d the formation into a plenum within the drill bit and drilling fluid circulating through the drill bit flushes fluid past apertures formed in front of the cutters to remove the formation cuttings.
U.S. Pat. No. 5,957,227 to Besson et al. and jointly assigned to the assignee of the present invention, discloses a drill bit incorporating blades which have primary and secondary cutting elements, such as PDC cutters, mounted so as to have a negative rake angle. Each of the blades are provided with tunnels or channels having a small opening located intermediate the primary cutters and the secondary cutters with respect to the direction of rotation of the drill bit. Each tunnel or channel is further provided with a larger dimensioned outlet positioned behind the secondary cutters. In one embodiment, the tunnels or channels are provided with nozzles for emitting fluid within the channel to carry formation cuttings toward the channel outlet.
While it is known that flushing fluid in proximity of conventional type cutting elements typically having negative rake angles works effectively to disperse formation cuttings away from the formation as the drill bit is in operation, the art continues to seek further advantages and efficiencies which may be gained by introducing drilling fluid proximate the cutting surfaces of cutting elements which may incorporate non-conventional configurations and which may incorporate positive rake angles to more efficiently remove formation cuttings away from the cutting elements and the bit.
In accordance with the present invention, a cutting element for use in a rotary drill bit is configured to enhance the stress state of the cutting element to accept loading imposed on the cutting element during drilling by reducing tensile loading of the cutting element and enhancing compressive stresses. The cutting element, when positioned in a drill bit body, facilitates placement of the superabrasive cutting member in suitably high compression during operational loading conditions while allowing the superabrasive cutting member to be positioned at a positive rake angle, including high positive rake angles, to prevent or lessen damage to the cutting element and to lessen cutting loads. The cutting element may, most suitably, be positioned in a drill bit structured with passageways generally in alignment with the cutting element so as to further assist the cutting element to direct formation chips away from the bit body.
Cutting elements of the present invention comprise a cutting member made of a suitable superabrasive material, such as polycrystalline diamond or cubic boron nitride. The cutting member may be formed in any known manner, including employing known high-temperature, high-pressure (HTHP) techniques of constructing PDC elements. Because of the unique shape of the cutting element, however, a more suitable method of forming the cutting member may be a chemical vapor deposition (CVD) or diamond film process as described in U.S. Pat. No. 5,337,844 to Tibbitts, the disclosure of which is incorporated herein by reference.
Superabrasive cutting members embodying the present invention preferably have a leading edge positioned to contact a formation for cutting and a three-dimensional arcuate curette or scoop-like, surface positioned rearward of the leading edge to direct formation chips away from the leading edge of the cutting element. The unique configuration of the cutting member allows the cutting element to be positioned in a drill bit body at a positive rake angle including high positive rake angles to shear chips or cuttings from the surface of the formation. As such, the cutting element is beneficially positioned to enhance compressive stresses in the cutting element and to prevent or lessen unwanted stresses in the cutting element and bit.
The three-dimensional scoop-like surface, as viewed in lateral cross-section of the cutting element, directs formation chips away from the leading edge of the cutting element. The cutting elements may, most suitably, be positioned in a drill bit body which is configured with passageways through which formation chips produced by the cutting element are flushed away from the leading edge of the cutting element through the passageway and are eventually discharged from the passageway so that the formation chips can further be circulated up the annulus between the drill string and the well bore.
Cutting elements of the present invention are suitable for use in known drill bit configurations, such as the bit configuration disclosed in U.S. Pat. No. 5,199,511 to Tibbitts, et al. or the drill bit configuration disclosed in U.S. Pat. No. 4,883,132 to Tibbitts.
Cutting elements of the present invention may also be attached to a drill bit as disclosed and described herein where passageways are formed through the drill bit body and in alignment with which the cutting element is placed to direct the sheared chips toward and through the associated passageway. The bit body disclosed herein is also preferably constructed with fluid passages positioned to deliver fluid to the passageways to facilitate flushing formation chips from the passageway and away from the bit body.
A superabrasive cutting element configured in accordance with the present invention may be formed or disposed directly to the bit body during construction or formation of the drill bit. In an alternative embodiment, the cutting element may comprise superabrasive material formed to a substrate, backing or stud by, for example, an HTHP or CVD process. The substrate of the cutting element may then be secured to the bit body by known techniques, such as brazing or furnacing. The substrate of the compact may, most suitably, be made of a carbide material such as tungsten carbide or other carbide material.
Cutting elements in accordance with the present invention may be configured in a variety of ways to provide a leading edge and a three-dimensional arcuate, curette-like, or scoop-like surface which preferably partially or fully curves toward itself to create a hollow region or volumetric cavity within the cutting element in which formation chips are guided through upon the formation chips being sheared by the leading edge of the cutting element. For example, a cutting element may be configured as a truncated frustum or hollow pyramid where the small or truncated end provides a first end defining the leading edge of the cutting element. The base of the pyramid defines a second end which is spaced apart from the first end and is configured for positioning in or toward the bit body of a drill bit. A three-dimensional scoop surface extends between the first end or leading edge and the second end of the cutting element and is positioned rearward of the leading edge to direct formation chips away from the leading edge. The cutting element, in longitudinal cross-section, may have the same thickness measurement at the leading edge as measured at the second end. In the alternative, a cutting element may have a greater thickness dimension at the second end than at the first end or leading edge, thereby giving the cutting element a wedge shape in longitudinal cross-section. The leading edge of the cutting element may be substantially linear (i.e., straight-edged) or can be curved.
Cutting elements embodying the present invention may also be formed as a truncated hollow cone where the small or truncated end of the cone defines the first end or leading edge of the cutting element and the base of the truncated cone forms the second end. In some embodiments, the element may be configured as a truncated pyramid or truncated cone, or any other suitable geometry. Alternatively, the cutting element may be formed as a longitudinal section (e.g., substantially one-half of the truncated cone) of such truncated pyramid, cone or other suitable shape.
The drill bit configuration as disclosed herein may also preferably be provided with depth-of-cut limiting structures to limit the amount of formation in which the cutting elements engage and remove chips or cuttings from the earth formation. The depth-of-cut limiting structure or structures may take any suitable form, a number of examples of which are disclosed herein. Furthermore, the drill bit configuration as disclosed herein is preferably provided with internal passages in fluid communication with an internal plenum within the drill bit body. The internal passages terminate at fluid discharge ports positioned within proximity of the disclosed cutting elements. The fluid discharge ports can be positioned aft of the cutting elements and positioned within the interior of the previously mentioned passageways to introduce drilling fluid directly therein to further assist the removal of formation chips away from the leading edge of the cutting elements. Alternatively, or in combination, fluid discharge ports may be located forward of the disclosed cutting elements and thus external of the preferably provided passageways.